The present invention relates to semiconductor laser devices, and particularly relates to blue-violet semiconductor laser devices that perform self-oscillation.
Semiconductor laser devices, whose current-optical output power characteristics exhibit excellent linearity and which emit highly monochromatic intense light, can focus laser light to a small spot.
Thus, semiconductor laser devices are used as light sources in optical pickups that drive equipment for recording media, such as optical discs and magneto-optical discs, that are written and reproduced by application of light. In recent years, semiconductor laser devices have started being used in drives for high-density recording media, such as blue DVDs (Digital Versatile Discs). In particular, devices as light sources using group III nitride semiconductor laser devices are being developed.
When laser light emitted from a semiconductor laser device is reflected off the optical disc, and re-enters the facet of the semiconductor laser device as return light, the return light causes noise. To reduce such return light noise, a method is adopted in which when a signal is reproduced, a high-frequency current is superimposed on the semiconductor laser device so as to make the oscillation spectrum be multimodal, thereby reducing the coherence of the laser light, and thus, reducing return light noise.
As a method for reducing return light noise, there is a known method in which a region (called a saturable absorption region) having light absorption effect is formed around a light amplification region and a gain region (called an absorption region) in an active layer so as to perform self-oscillation. During self-oscillation, the effective refractive index in the waveguide changes to cause the oscillation wavelength to fluctuate, thereby reducing the coherence of light, and thus, reducing return light noise.
For example, Japanese Laid-Open Publication No. 2000-286504 (hereinafter referred to as “Patent Document”) describes a nitride semiconductor self-oscillation laser device that includes a light amplification region and a saturable absorption region serving as a light absorption region.
FIG. 12 illustrates a cross-sectional structure of the nitride semiconductor laser device described in Patent Document. As shown in FIG. 12, the conventional nitride semiconductor laser device includes an n-type contact layer 102, an n-type clad layer 103, an active layer 104, a p-type clad layer 105, an n-type current confinement structure 106, and a p-type contact layer 108 sequentially stacked on the principal surface of a substrate 101 made of sapphire.
The p-type clad layer 105 includes a flat portion 105a, a lower stripe portion 105b, and an upper stripe portion 105c. The flat portion 105a is formed so as to cover the top surface of the active layer 104. The lower stripe portion 105b having a width of W2 is formed so as to protrude upwardly from the central part of the flat portion 105a. The upper stripe portion 105c having a width of W1 is formed so as to further protrude from the central part of the lower stripe portion 105b. That is, the lower stripe portion 105b and the upper stripe portion 105c are formed in such a manner that the width W1 is smaller than the width W2.
An n-side electrode 109 is formed on the exposed part of the n-type contact layer 102, and a terminal 110 is formed on the p-type contact layer 108.
The nitride semiconductor laser device thus structured is regulated by the width W1 of the upper stripe portion 105c, and a current flowing from the p-type clad layer 105 to the active layer 104 is controlled so as not to expand laterally. Consequently, a current injection region of a size corresponding to the width W1 of the upper stripe portion 105c is formed in the central part of the active layer 104. Also, since the width W2 of the lower stripe portion 105b is greater than the width W1 of the upper stripe portion 105c, the emission spot has a width corresponding to the width W2 of the lower stripe portion 105b, and a saturable absorption region is formed around the current injection region. As a result, in the active layer 104, the current injection region and the saturable absorption region interact with each other, thereby performing self-oscillation, and thus achieving a pulsed optical output.
In this self-oscillation semiconductor laser device, the intra-active-layer optical gain region (having a width of G) occurring due to expansion of current is narrowed as much as possible, while the spot size (having a width of S) of the waveguide is set to a relatively large size. When the relationship S>G is satisfied, the difference therebetween functions as a saturable absorber, causing self-oscillation to occur.
Thus, the waveguide satisfies the above relationship as an intermediate waveguide between an index-guide laser device and a gain-guide laser device. Producing sufficient saturable absorption effect is important to maintain stable self-oscillation. The effect of the saturable absorber is effectively increased when the differential gain (∂G/∂n where G represents the optical gain, and n represents the injected carrier concentration) of the emission optical gain region in the central part of the active layer 104 (i.e., saturation at the time of laser oscillation) is low, and the differential gain of the saturable absorber is high, that is, when the difference therebetween is large. Thus, the differential gains and the magnitudes thereof are important as self-oscillation conditions. For the active layer 104, a multiple quantum well (MQW) structure is often employed.
In order to achieve stable occurrence of self-oscillation, the following two items need to be satisfied.
(1) A difference between the differential gain in the emission region in the active layer and that in the saturable absorption region should be high, and the differential gain in the emission region should be easily saturated.
(2) The light absorption effect in the saturable absorption region should be considerable.
FIG. 13 shows the relationship between a typical optical gain G and an injected carrier concentration n. To achieve stable occurrence of self-oscillation, the conditions in the above-described items (1) and (2) need to be satisfied. However, in a case in which the differential gain in the emission region is not likely to become saturated, it is difficult for self-oscillation to occur stably.
FIG. 14 qualitatively shows the relationship between the number N of quantum wells in a MQW structure and an injected carrier concentration related to the optical gain. As the number N of quantum wells is increased, it becomes difficult for the optical gain to become saturated, and thus, self-oscillation is difficult to achieve.
That is, the smaller the number N of quantum wells is, the more easily the gain G of the emission region becomes saturated. Hence, as compared with an active layer having a MQW structure in which the optical gain is hardly saturated, the optical gain is easily saturated as in a bulk active layer. Therefore, increasing the effect of saturation leads to the reliable self-oscillation.